The goal of plant breeding is to develop new, unique and superior cultivars. Theoretically, a breeder can generate billions of different genetic combinations via crossing, selfing and selection. A breeder has no direct control at the cellular level. Therefore, two breeders will never develop the same line, or even very similar lines, having precisely the same traits. Descriptions of breeding methods that are commonly used for different traits and crops, as well as specifically for tomato, can be found in one of several reference books (e.g., Allard, R. W. (1960) Principles of Plant Breeding; Simmonds, N. W. (1979) Principles of Crop Improvement; Sneep, J. et al., (1979) Tomato Breeding (p. 135-171) in: Breeding of Vegetable Crops, Mark J. Basset, (1986, editor), The Tomato crop: a scientific basis for improvement, by Atherton, J. G. & J. Rudich, (1986, editors); Plant Breeding Perspectives; Fehr, (1987) Principles of Cultivar Development—Theory and Technique).
The method chosen for breeding or selection depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the cultivar (i.c. variety) used commercially (e.g. F1 hybrid, or an open-pollinated variety). The complexity of the inheritance influences the choice of breeding method. One simple method of identifying a superior plant is to observe its performance relative to other experimental plants or to a widely grown standard cultivar, and to observe its performance in hybrid combinations with other plants. If single observations are inconclusive for establishing distinctness, observations in multiple locations and seasons provide a better estimate of its genetic worth. Proper testing and evaluation should detect any major faults and establish the level of superiority or improvement over current cultivars.
The development of commercial tomato hybrids requires the development of homozygous inbred parental lines. In breeding programs desirable traits from two or more germplasm sources or gene pools are combined to develop superior breeding lines. Desirable inbred or parent lines are developed by continuous selfing and selection of the best breeding lines, sometimes utilizing molecular markers to speed up the selection process.
Once the inbreds that give the best hybrid performance have been identified, the hybrid seed can be produced indefinitely, as long as the homogeneity and the homozygosity of the inbred parents is maintained. A single-cross hybrid is produced when two inbred lines are crossed to produce the F1 progeny. Much of the hybrid vigor exhibited by F1 hybrids is lost in the next generation (F2). Consequently, seed harvested from hybrid varieties is not used for planting stock.
There are numerous steps involved in the breeding and development of any new and novel, desirable plant germplasm with superior combining ability. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals and the definition of the best breeding method to reach those goals. The objective is to combine in a single hybrid variety an improved combination of desirable traits from the parental germ plasm. Important characteristics may include higher yield, better flavor, improved color and field holding ability, resistance to diseases and insects, tolerance to drought and heat, along with characteristics related to hybrid seed yields to lower the cost of hybrid seed production.
Tomato is a very important crop in all continents of the world. Several plant species associated with the Solanum group have been familiar to mankind since ancient times, and are of great agricultural importance. Solanum species have a general adaptation to variable climatic growing conditions. Tomato (Lycopersicon esculentum L.) belongs to the Solaneaceous family. All varieties in the species esculentum are self-pollinating. Most other species in the genus Lycopersicon are cross-pollinating. Cross-pollination is affected by insect vectors, most commonly by the honey- or bumblebees. Tomato, like most other Lycopersicon species, is highly variable. Variability in populations is desired for wide adaptation and survival. Tomato is adapted to warm summer growing conditions, but can also be grown in heated greenhouses under winter conditions. The introduction of hybrid cultivars in the 1950's provided a magnitude of benefits like increased yield, better holding ability, adaptation to expanded growing seasons through the use of protected cultivation and improved disease resistance, which resulted in large-scale production of tomato as a commercial crop.
The goal in tomato breeding is to make continued improvements in hybrid tomato yields, in other horticultural characteristics, as well as in quality traits, in order to meet continuous demands for better tomato cultivars in different growing regions of the world.